Hello,
at
http://libvirt.org/formatdomain.html#elementsNICS there are three
examples of interfaces with a particular MAC address specified. I
understand the provided mac address is more a placeholder than a real
value; but since '11:22:33:44:55:66' seems to be a multicast address[1],
I would suggest providing '00:11:22:33:44:55' as example.
Best regards
[1]
http://en.wikipedia.org/wiki/MAC_address#Address_details